This invention relates generally to transmission circuitry for use in telephone communication systems and more particularly to a new transmission bridge useful in providing voice communication.
Transmission bridges have long been utilized to provide talking paths in 2-wire telephone switching networks and, therefore, have been designed to pass AC signals, while isolating DC voltages which are provided to perform a number of supervisory functions. The design of transmission bridges has been quite functional in the past. Thus, such bridges have in general been basically transformers (for example) which pass AC signals and are sized to suit particular applications. The tip and ring lines in both the primary and secondary sides of the transformer have been connected through the windings to ground and the negative battery, respectively. Capacitors have been installed, in both the primary and secondary between windings connected in the tip circuit and windings connected in the ring circuit in order to provide the required DC isolation.
In older transmission bridges high impedance iron core wound relays were often used in order to satisfy the transmission characteristics of the bridges. In newer bridges, in which relatively lower impedance reed relay sensors (for example) are utilized, repeat coils, which have a large inductance, have been connected between the tip and ring circuits. The use of repeat coils has also permitted the necessary impedance transformations (for example 900:600 ohms between line and trunk circuits).
Such transmission bridges generally have provided adequate service; however, they have exhibited several deficiencies and are expected to be inadequate to meet a number of future requirements. With the advent of direct distance dialing, some line-to-trunk and trunk-to-trunk transmission bridges are required to transmit rotary dial pulses. Often existing transmission bridges have distorted these pulses so that the signals which pass through the bridges have resulted in incorrect connections or no connections at all. Such bridges probably will be inadequate for future data transmission requirements. More important for the present, existing transmission bridges generally exhibit voice transmission characteristics which are less than completely desirable. Generally, the insertion loss in existing bridges is greater than is desired. The frequency of voice signals generally lies in a range between 300 Hz and 3,000 Hz. Typically, the response of an existing transmission bridge is of the order of 2-3 dB down at 300 Hz from that at 1,000 Hz and the response at 3000 Hz is correspondingly further down.
In order to provide optimum transmission characteristics, a transmission bridge should have a low audio insertion loss and exhibit a frequency response curve which is substantially constant over the frequency range corresponding to voice signals. At the same time the response should drop off sharply below 200-300 Hz in order to attenuate the low frequency noise signals which are often present on telephone transmission lines.
Such a transmission bridge should protect the equipment to which it is connected from large current surges and should accommodate both rotary and pushbutton dialing. In addition, such a transmission bridge should be simple, inexpensive and easily manufactured.
Accordingly, its is an object of the present invention to provide a new and improved transmission bridge which exhibits low audio insertion loss and a flat audio frequency response curve and provides rejection to low frequency noise signals.
Another object of the invention is to provide such a transmission bridge which causes very little distortion, thereby accommodating both rotary dial and pushbutton signalling, and demonstrates high longitudinal balance.
Still another object of the invention is to provide such a transmission bridge which provides required impedance transformations and high echo return loss.
A further object of the present invention is to provide such a transmission bridge which protects against current surges due to lightning induction and is simple and inexpensive to manufacture.